State Atty. Gen. Bill Lockyer will take the lead on behalf of the state, urging that the high court uphold the system. Davis' friend-of-the-court brief filed Thursday adds weight to the case, given that he is the state's highest-ranking elected official.

Davis, a moderate, was one of the few officials who supported the 1996 initiative pushed by Rep. Tom Campbell (R-Palo Alto) to create the system, which permits voters regardless of party affiliation to vote for the candidates of their choice in primary elections.

"The governor believes and I do too that independent, nonaffiliated voters are pretty much the wave of the future," said Garry South, Davis' chief political advisor. "Whichever party finds a way to . . . make them welcome is going to be the party of the future."

Campbell, the Republican nominee for the U.S. Senate seat held by Dianne Feinstein, welcomed Davis' brief, a campaign aide said. Campbell plans to file his own written arguments. The high court is scheduled to hear arguments in the case next month.

The Democratic and Republican parties are urging that the high court overturn the 1996 initiative that created the open primary system, contending that it violates their 1st Amendment right to nominate their own standard-bearers by permitting voters who do not belong to their parties to cast some of those votes.

"We believe our membership ought to be determined by our own ideology," said Democratic Party Chairman Art Torres. However, Torres said the party has "no quarrel" with Davis, because the governor has supported the open primary for years.

To accommodate the major parties, California Secretary of State Bill Jones agreed to code the March primary ballots to allow the parties to count only their members' votes when awarding delegates to their presidential nominating conventions this summer.

In his friend-of-the-court brief, Davis said the system led to increased voter participation in 1998 and in the primary held earlier this month. He noted that California is not a strongly partisan state, and that a third of all new voters in the past decade have declined to state a party affiliation.

"The blanket primary now encourages candidates to appeal to a larger section of the electorate and thus expands the scope of debate beyond limited partisan concerns," Davis said in his brief.

The Democratic governor pointed out that the initiative, Proposition 198 of 1996, won by a wide margin, and that courts should not lightly overturn popular ballot measures. Courts, the brief says, should not "become embroiled in the passions of the day and assume primary responsibility in choosing between competing political pressures."

"Constitutional issues cannot be resolved on the basis of public opinion," the brief says. "However, when a question is debatable, such as the relative benefits of the blanket primary, the voters' widespread belief as to the merits must be considered."